Addition polymerizable silicone resins are widely used in many fields such as electronics, health care and automotive applications. The polymerizable resins are cured as a two-part system using a hydrosilation reaction. A platinum catalyst is used in one part, the catalyst side, and a hydrogen terminated polydimethylsiloxane (HPDMS) in the other part, the base side, while both sides contain vinyl terminated polydimethylsiloxanes (PVDMS) resins. When these materials are cured at room temperature, they are referred to as room temperature vulcanized (RTV). The most common RTV materials are typically offered as a 10:1 ratio base/catalyst, such as RTV630 (GE Silicones), while some other RTV materials are offered at a 1:1 ratio, such as RTV6428 (GE Silicones). Various working times are required depending on the application from 2 minutes to several hours and may involve a heat curing step above ambient temperature. The working time is controlled with a retarder or inhibitor mixed with the catalyst component, such as an amine or acetylenic compound.
Another class of addition polymerizable silicone resins are the liquid silicone rubber (LSR) materials prepared through the liquid injection molding (LIM) process. The LSR materials are cured at a temperature of 120° C.–180° C. in a mold injected to after mixing. The mixture includes a retarder mixed with the catalyst component, such as an amine or acetylenic compound, that allows the hydrosilation reaction to occur at the mold temperature only.
Both the RTV and LSR types of formulations suffer from the shortcomings of the hydrosilation mechanism. These shortcomings include: (1) deactivation of the platinum catalyst by sulfur or other nucleophilic impurities; (2) high shrinkage, approximately 1%, due to the high reduction of free volume upon polymerization; (3) high cost of platinum metal needed for catalysis; (4) high cost of HPDMS and PVDMS resins; (5) requirement of two different resins to be employed, namely vinyl and hydrogen terminated; (6) undesirable hydrogen evolution from the decomposition of the hydrosiloxane cross-linkers that typically are present in these systems; and (7) vinyl functionalized PDMS resins have a low hydrocarbon content in the main chain after polymerization due to the presence of only an ethyl spacer, which may lead to a relatively high dielectric constant, which is an undesirable property for some electronic applications.
A new type of polymerization system has been recently developed that may potentially be used to replace addition-curable silicones and platinum catalysts in a wide variety of applications to thereby avoid the shortcomings of the hydrosilation mechanism discussed above. In this new metathesis reaction system, curing is achieved by a ring-opening metathesis polymerization (ROMP) mechanism. Metathesis is generally understood to mean the metal catalyzed redistribution of carbon-carbon double bonds. The polymerizable composition comprises a resin system that includes functionalities or groups that are curable by ROMP together with a metathesis catalyst, such as a ruthenium carbene complex. However, to efficiently utilize ROMP to prepare polymers, there is a need to control the progress of polymerization, particularly for molding applications.
In commonly-owned U.S. Pat. No. 6,649,146, a two-part room temperature ROMP-curable formulation containing siloxane polymers and fillers was described as usable as a dental impression material. The catalysts described therein are ruthenium carbene complexes containing phosphine ligands. These catalysts, however, are air sensitive because the phosphines can dissociate and oxidize, thereby leading to reduced shelf life. An alternative highly active ruthenium carbene complex that does not contain phosphine groups is described in Hoveyda et al. U.S. Patent Publication No. 2002/0107138 A1. This alternative catalyst has a good air stability profile. However, when this alternative catalyst is substituted for the catalysts containing the phosphine ligands, the composition exhibits a very short working time after mixing, on the order of 20 seconds, which makes the use of these compositions impractical in many applications, such as dental impression materials. There is thus a need to provide a room temperature ROMP-curable formulation that has good air stability as well as a longer working time after mixing.
In addition to ROMP, other metathesis reaction systems utilize metathesis catalysts, for example ring closing metathesis, acyclic diene metathesis polymerization, ring opening metathesis and cross metathesis. There is further a need for controlling the progress of reaction in these other metathesis reaction systems.